The invention relates to an optical amplifier which amplifies signals at a certain signal wavelength and consists of an active optical wave guide of simple mode geometry at the signal wavelength, and a light source, e.g. a pump laser for obtaining inverted population between the energy levels involved and coupling devices for coupling pump light and signal light in the active optical wave guide. The wave guide is doped with active ions which are substantially positioned in a cylindrical shell-shaped area around the axis of symmetry of the wave guide.
The use of optical amplifiers constitutes a technically interesting field, because optical power can be amplified with these directly in an optical fiber or wave guide without conversion of the optical energy to electric energy.
Such optical amplifiers are used or may possibly be used in fiber optical communications systems, for fiber lasers, for fiber optical sensor systems or the like. Also amplifiers manufactured in plane wave guide technology can be used within the above-mentioned fields.
Optical amplifiers and in particular fiber optical amplifiers operate by the process "stimulated emission", where a material can emit light with the same wave vector and phase as incoming light, it being energy-supplied (pumped) with light from a strong pump source, typically a laser having a lower wavelength than the signal. Amplification in a material capable of amplifying by stimulated emission depends on the dipole moment of the quantum transition considered, on the wavelength of the signal and on the size of the relative population of the two quantum states directly involved. Where the upper one of the two levels has a greater population than the lower one, so-called inverted population, it is possible to obtain amplification. This inverted population is generated via absorption of pump light.
The materials capable of forming the basis for stimulated emission can generally be divided into two groups: 3-level systems and 4-level systems.
The European patent application 368 196 discloses a fiber amplifier in which the active Er.sup.3+ ions are distributed along the axis of symmetry of the fiber in a cylinder shell which is contained completely within the core region. The optical fiber amplifier is pumped with energy distributed on several transversal wave types, and the cylinder shell is therefore spaced from the axis of symmetry corresponding to the distance where the pump light has maximum intensity. This design is not optimal for amplification of light by means of 4-level systems with amplified spontaneous emission (ASE) at lower wavelengths than the signal.
The invention also concerns a method of amplifying an optical signal with an optical amplifier, which is stated in claim 1. Hereby it is possible e.g. to obtain discrimination between the formation of spontaneous photons at a shorter wavelength (e.g. 1050 nm) by formation of signal photons at signal wavelength (e.g. 1340 nm) by stimulated emission. Amplification of signal photons will be increased considerably hereby, because the pump light creating inverted population for the levels involved is utilized better for the 1300 nm transition than for the 1050 nm transition. The invention can e.g. be worked in connection with effective amplifiers based on Nd.sup.3+ doped ZBLAN glasses, but can be utilized by any 4-level optical amplifier to which it applies that the signal wavelength is greater than the wavelength of the dominating amplified spontaneous emission.
One of the crucial problems of e.g. Nd.sup.3+ doped fibres is thus that a large amount of ASE is formed at considerably lower wavelengths than the wavelength of the signal to be amplified. The invention reduces this problem by using a special design of the active fiber. When the fiber is manufactured such that doping is in a ring around the axis of symmetry, the formation of the ASE having a low wavelength will be reduced relatively more than the stimulated amplification of signal light, and amplification at the signal wavelength will therefore be increased considerably. Removal of the doping from the center of the core reduces the amplification of the light both at the short and at the long wavelength, but the decisive point is that amplification of the light having the short wavelength is reduced relatively more than the long-waved light.
Since the undesirable ASE has a considerably lower wavelength than the signal, the region occupied by this ASE in the fiber, e.g. described by the dot size D.sub.ASE, will be considerably smaller than the dot size d.sub.S of the signal. Amplification of the short-waved ASE therefore diminishes more rapidly than the amplification of the signal if the doping of the active ions is moved away from the center of the wave guide and is positioned in a cylinder shell-shaped area concentric with the axis of symmetry of the wave guide. The invention utilizes this circumstance for suppressing the undesirable ASE to achieve greater amplification of the desired signal. Another object of the invention is to provide an optical amplifier having improved efficiency over the prior art.
This object is achieved by an optical amplifier having the features defined in the characterizing portion of claim 2. A considerably part of the pump energy is hereby absorbed in parts of the active optical wave Guide, where the proportion between formation of spontaneous emission at considerably shorter wavelengths than the signal wavelength and the formation of spontaneous emission around the signal wavelength is greatest. Thus, also the proportion between amplification of the spontaneous emission at considerably shorter wavelengths than the signal wavelength and amplification of the signal light is greatest.
The active optical wave guide is doped with ions of transition metals or metals from the groups of Lanthanides or Actinides.
Claim 3 defines the active optical amplifier having a wave guide which, according to the invention, is to be produced with the doping which does not coincide with the core, if the active ion is of the 4-level type. Improved amplification will be obtained by moving the doping away or partly away from the core. If the inner radius of the doping is called a.sub.1 and the outer radius of the doping is called a.sub.2, these diameters will be related to the core radius a in the following manner: EQU a&lt;a.sub.2 a.sub.1 &lt;a.sub.2.
Claim 4 defines an amplifier consisting of three main elements: the active wave guide, coupling devices for coupling pump light and signal light as well as one or more pump sources. In case of Nd.sup.3+ doping the pump source is to have a wavelength around 0.795 .mu.m, while other active ions demand other pump wavelengths. The pump wavelength will be smaller than the signal wavelength under normal conditions.
Claim 5 defines an amplifier having an active wave guide doped with two or more active ions having overlapping amplification regions. This may be an advantage when using ions which themselves cannot cover the entire interesting amplification region.
Claim 6 defines an amplifier having two amplification regions, e.g. coinciding with the most interesting transmission fields within fiber optical communication. Such an amplifier may be produced using two different active ions having different amplification regions, e.g. Nd.sup.3+ for the 1300 nm region and Er.sup.3+ for the 1550 nm region.
Claim 7 defines an embodiment of an amplifier for the situation where such a double ion doping is necessary to obtain amplification in two wavelength regions. Here doping is performed with a 4-level ion in a cylinder shell-shaped area and with a 3-level ion in the core area, which is done to obtain the greatest amplification.
Claim 8 defines preferred ion types, which have been found to be more effective than others. Ions operating according to 4-level systems are preferred, because active optical wave guides made with doping of 4-level ions do not exhibit ground state absorption. The ions Nd.sup.3+ and Pr.sup.3+ are of special interest for the region around 1300 nm, which have both exhibited good amplification properties in this region.
Claim 9 shows that the host glass for the production of the active wave guide is of importance for the amplification properties, because the host glass affects the energy of the quantum states of the active ions. The host glass thus influences absorption wavelengths and emission wavelengths. Host glass based on SiO.sub.2 is of special importance, because ordinary transmission fibers for optical communication are made of this material.
Claim 10 describes host glasses made on the basis of metal fluoride glasses, e.g. ZBLAN, which have another influence on the absorption and emission of the active ions than SiO.sub.2 based glasses. ZBLAN glasses are particularly interesting, because it has been found possible to make Nd.sup.3+ doped fibres having considerable amplification around 1300 nm in this material.
Claim 11 defines a preferred embodiment of the active optical wave guide. One of the most interesting uses of optical amplifiers is in connection with fiber optical transmission systems. In this case it is extremely advantageous that the active wave guide is an optical fiber. Then the active ions will lie like a cylinder shell concentrically around the axis of symmetry of the fiber and along the fiber core. The active doping can either overlap the core or be positioned right out in the cladding of the fiber.
Claim 12 defines an amplifier having an active wave guide based on plane wave guide technology. For certain uses, and especially if a particularly inexpensive amplifier component is desired, it is advantageous to make the amplifier as an integrated optical component. Then the wave guides, and thereby also the active optical wave guide, will have to be made in plane wave guide technology with wave guides having a substantially rectangular cross-section. Examples of substrate materials for such integrated components are Si/SiO.sub.2, LiNbO.sub.3 and III-V semiconductor materials.
Claim 13 defines the wave type of the pump light. If the pump light passes through the active wave guide in the fundamental transversal wave type, advantages are obtained in particular for 3-level ions with active doping in the core, but also 4-level ions having cylinder-symmetrical doping can be excited. It is important in this case that the 4-level ions are doped in a cylinder shell for the greatest amplification to be achieved.
Claim 14 defines the use of pump light with several wave types. For 4-level ions even greater amplification can be obtained if the pump light passes the active wave guide in several transversal wave types. The reason is that the higher order wave types give rise to a greater intensity of pump light away from the axis of the wave guide and thus have better overlap with the cylinder shell-shaped doping.
Claim 15 describes a preferred structure of the active optical fiber. Calculations show that an Nd.sup.3+ doped ZBLAN fiber can be optimized to being particularly amplifying if doping is arranged in a region defined by: EQU a.sub.1 /a is in the range [0.6; 1.4] EQU a.sub.2 /a is in the range [0.85; 4.0]
Claim 16 defines the use of the amplifier described in claims 2-15 as a light source. When reflection devices, such as mirrors, couplers, reflection screens, etc., are arranged at one or both ends of the active wave guide, part of the light emitted from the amplifier will be returned to the amplifier and amplified additionally. Using such a reflection device broad spectrum optical sources having a short coherence length can be produced. Using reflection devices at both ends of the active wave guide, a laser can be produced, the laser cavity being defined by these two reflection devices.